UK light heavyweight Buatsi secured a victory yet unimpressive start to life under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a competitive ten-round bout at the Co-op Live Arena.
But the challenger found success, delivering effective single shots in a fight where the techniques never quite clicked, with frequent clinching spoiling the action.
The scoring judges judged it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest showing but what defines champions? They win," the victor stated in the arena as he was met by negative reactions from the crowd.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a 20th pro success.
Various analysts felt the challenger, 31, deserved the decision but the British boxer suffered the second loss of his 28-fight professional journey.
The Champion Earns Win in Dull Affair
Buatsi's progress has slowed in recent years through management issues and extended periods of inactivity.
Earlier forecast as the future of British boxing, he now is positioned trying to realize his potential - perhaps in what appears to be a last chance - under recent support.
He opened tentatively, with the two boxers willing to overcommit in a calculated opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in two years ago after breaking his hand during the bout, was making just his second fight at the higher weight class.
By the middle rounds, Parker had kept it close, making life awkward for Buatsi, who worked behind a solid defense to edge the more effective stanzas.
The crowd's patience wore thin because of regular grabbing, with Parker slipping to the canvas several times during close exchanges.
The underdog ended stronger, but multiple periods were challenging to assess. Although, ex-champion Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
The victor moves on, but with the likes of hard hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Tribute Inspires All-Manchester Contest
In the co-main event, Arthur overcame Rea by close verdict in a terrific all-Manchester battle to capture the European light-heavyweight championship.
The arena became quiet for a deeply poignant homage ahead of the contest as 10 bells tolled in memory of the city's native Ricky Hatton, who passed away 46 years old in September.
It was the initial significant broadcast match card in the urban center since the passing of Hatton. As the final chime ended, the silence became a roar of "there's just one Hatton".
The competition inside the ropes matched the emotion. The eventual winner was sent to the canvas by a left hook squarely on the chin in the initial stage, rising shakily to his footing on the eight count.
But the traditional red shorts of the fighter soon pressed ahead again. On the opposite side, the opponent - in rival City blue - was marked up soon, noticeable damage appearing under his eye as the duo exchanged powerful shots.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, fought fire with fire with effective uppercuts as the pace rarely dropped over 12 rounds.
Arthur barely secured the result with scoring tallies of even score, 115-113 and 115-112, but the two athletes were relentless and courageous, exactly what their idol the legend represented.
